There is some confusion in your post as the G50 is not a Getrag it is Porsche transaxle. The G50 comes in several variants, some beefier than others witgh larger R&P. I run the beefier G50/52 in an Ultima and the inversion process and mods required to handle 600HP (oil spray bar, billet side plates etc.) pushed the box cost to $14K.
Why are you so keen to move away from a ZF? If you do want something other than a true ZF then IMO the choice would be the Quaife ZFQ, dimensionally the same as the ZF with Porsche internals and cost is back under $10K at current GBP exchange rates.
Julian
